Fully quantitative imaging of target response to therapy offers the potential to provide early assessment of treatment efficacy, which may lead to individually tailored therapeutic plans and improved outcomes. Current guidelines for assessing the response of a target, such as a tumor, to therapy still rely heavily on measuring target volume via anatomical imaging. Thus, assessment of target response by CT scan or MRI is still the “gold standard” for target response evaluation. However, such anatomical changes are often delayed for weeks or months after initiation of treatment, and may not adequately reflect treatment efficacy. Real time non-invasive assessment of target response to treatment is therefore currently a major challenge in a wide variety of diseases, such as cancer and central nervous system diseases.
Detection of target-related cellular processes through molecular imaging may potentially address this need, and positron emission tomography (PET) with markers of glucose metabolism (e.g., 18F-fluorodeoxglocose; [18F]-FDG), cell proliferation (e.g., 3′-deoxy-3′-18F-fluorothymidine; [18F]-FLT), or amino acid uptake (e.g., [18F]-L-tyrosine) are increasingly utilized in oncological practice. All of these biomarkers detect molecular processes occurring in the viable cell, however they are limited by sub-optimal specificity, a small dynamic range of the observed changes, and/or limited applicability to slowly growing tumors.